1. Field of the Invention
The present invention relates generally to discharge tubes, and more particularly to a discharge tube that causes discharge to repeatedly occur between the discharge surface of an upper discharge electrode end and the discharge surface of a lower discharge electrode end, the discharge surfaces opposing each other at the center inside an airtight tube.
2. Description of the Related Art
For instance, Japanese Laid-Open Patent Application No. 10-335042 discloses a switching discharge tube (hereinafter, simply referred to as “discharge tube”) used in lighting circuits for an HID (High Intensity Discharge) lamp for vehicles, projector lamp, and a back lamp for rear-projection TVs. FIGS. 1 through 3 show a first conventional discharge tube 100A.
Referring to FIG. 1, the discharge tube 100A includes an airtight tube 10D, an upper discharge electrode 22, and a lower discharge electrode 24. The airtight tube 10D has a cylindrical shape. The upper discharge electrode 22 and the lower discharge electrode 24 are joined to the upper-end opening and the lower-end opening, respectively, of the airtight tube 10D.
Disk-like lid bodies 26 and 28 are formed integrally with the upper discharge electrode 22 and the lower discharge electrode 24, respectively. A metallized surface 40 is formed at each of the upper-end opening and the lower-end opening of the airtight tube 10D. Accordingly, the upper discharge electrode 22 and the lower discharge electrode 24 are joined to the airtight tube 10D by brazing the lid bodies 26 and 28 integrated with the upper and lower discharge electrodes 22 and 24, respectively, to the metallized surfaces 40 formed at the upper-end and lower-end openings of the airtight tube 10D. Referring to FIG. 3, lead wires 12 and 14 are connected to the lid bodies 26 and 28, respectively, so that the lid bodies 26 and 28 are connected to external circuits through the lead wires 12 and 14.
The upper discharge electrode 22 projects from the lid body 26 toward the center position of the airtight tube 10D. The end portion of the upper discharge electrode 22 is shaped like a cylinder of a small diameter. A discharge surface 23 is formed on the small-diameter cylindrical end portion of the upper discharge electrode 22 (hereinafter referred to as “upper discharge surface 23”). The upper discharge surface 23 includes a recess 27 for causing discharge to occur in a stabilized manner.
The lower discharge electrode 24 is structured in the same manner. The end portion of the lower discharge electrode 24 is shaped like a cylinder of a small diameter. A discharge surface 25 is formed on the small-diameter cylindrical end portion of the lower discharge electrode 24 (hereinafter referred to as “lower discharge surface 25”). The lower discharge surface 25 also includes the recess 27 for causing discharge to occur in a stabilized manner. In the discharge tube 100A, discharges occur in the space between the upper discharge surface 23 and the lower discharge surface 25. This space is hereinafter referred to as a “discharge gap 29.”
Referring to FIGS. 1 and 2, in the discharge tube 100A of the above-described structure, for instance, eight main discharge trigger wires 30 are formed along the axial directions of the airtight tube 10D (or the Y1 and Y2 directions of FIGS. 1 and 2) at equal intervals (with the same pitch W) on the inner sidewall of the airtight tube 10D. Each main discharge trigger wire 30 is spaced from the metallized surfaces 40 so as to be electrically isolated from the metallized surfaces 40.
FIGS. 4 and 5 are diagrams showing a second conventional discharge tube 100B. In FIGS. 4 and 5, the same elements as those of FIGS. 1 through 3 are referred to by the same numerals, and a description thereof is omitted.
According to the second conventional discharge tube 100B, two sub discharge trigger wires 20 as well as the eight main discharge trigger wires 30 are formed on the inner sidewall of an airtight tube 10E. Each sub discharge trigger wire 20 is formed at a center position of the eight main discharge trigger wires 30. That is, four of the main trigger wires 30 are provided in each of the two spaces between the paired sub discharge trigger wires 20.
Like the main discharge trigger wires 30, the sub discharge trigger wires 20 are formed along the axial directions of the airtight tube 10E (or the Y1 and Y2 directions of FIGS. 4 and 5). The upper or lower end of each sub discharge trigger wire 20 is electrically connected to the metallized surface 40 formed on the corresponding upper-end or lower-end surface of the airtight tube 10E.
Conventionally, the sub discharge trigger wires 20 and the main discharge trigger wires 30 are also formed along the axial directions of the airtight tube 10E (or the Y1 and Y2 directions of FIGS. 4 and 5) at equal intervals (with the same pitch W) as shown in FIG. 5. That is, when the distance (interval) between each adjacent two of the main discharge trigger wires 30 is W, the distance (interval) between each sub discharge trigger wire 20 and each of its adjacent main discharge wires 30 is also W.
FIG. 6 is a graph showing the results of a discharge (service) life test conducted to obtain changes over time in the discharge starting voltage of an initial discharge (hereinafter referred to as “initial discharge starting voltage FVs”) and the mean discharge voltage of second and subsequent discharges (hereinafter referred to as “mean discharge voltage Vs MEAN”) in the first conventional discharge tube 100A of the above-described configuration. In FIG. 6, the horizontal axis indicates the cumulative number of discharges (×10,000), and the vertical axis indicates discharge operation voltage (V).
In this test, the initial discharge starting voltage FVs and the mean discharge voltage Vs MEAN at the time of causing the discharge tube 100A to perform discharging after the discharge tube 100A was left in a completely dark place at −40° C. for a predetermined period of time were studied at each predetermined point. Specific test conditions are as follows:
(a) Operation Interval: a second of operation is followed by four seconds of quiescence (hereinafter, this is referred to as “one test cycle”). One hundred discharges are caused to occur in this one test cycle (five seconds), thus resulting in a discharge frequency of 100 Hz;
(b) Measurement Method: the discharge tube 100A is left in an environment of −40° C., and the test cycle is repeated until the cumulative number of discharges reaches a specified measurement number. When the cumulative number of discharge operations reaches each specified measurement number, the initial discharge starting voltage FVs and the mean discharge voltage Vs MEAN are measured and calculated.
Specifically, in the case of measuring data at a specified measurement number of 20,000, the test cycle is stopped when the number of times the test cycle is repeated reaches 200, and the discharge tube 100A is left as it is for an hour. Thereafter, the discharge tube 100A is caused to operate for one test cycle, and the initial discharge starting voltage FVs and the mean discharge voltage Vs MEAN are measured and calculated. When this measurement operation is completed, the test cycle is started, and is repeated until the next specified measurement number (for instance, 40,000). This operation is repeatedly performed until a specified measurement number of 2,000,000; and
(c) Power Supply Circuit for Test: a relaxation oscillator circuit including a capacitor and a coil is employed. In practice, a capacitor of 120 nF (50–150 nF) and a coil of 0.1 μH (0.1–5.0 μH) were employed. According to this relaxation oscillator circuit, when an electric charge is stored in the capacitor, a discharge tube connected thereto performs discharging so as to cause the electric charge to flow to ground. The capacitor, which has lost the electric charge, starts recharging, and when the electric charge is re-stored, the capacitor again discharges. This discharging is repeated in an operation period of one second. The cumulative number of times this discharging operation is repeated corresponds to the cumulative number of discharges of the discharge life test.
The discharge life test is conducted based on the above-described conditions. This discharge life test, which is conducted in a completely dark place in an environment of −40° C., is the severest one of the discharge life tests. This is because there is no effect of thermoelectrons in the environment of −40° C., nor is there any effect of photoelectrons in the completely dark place, thus making it difficult for discharging to occur. A brief description is given below of the effect of photoelectrons and the effect of thermoelectrons.
The effect of photoelectrons refers to the effect that the discharge characteristic of a discharge tube is made faster by photoelectrons. That is, photoelectrons are constantly emitted from the light source of, for instance, an illuminator, so that sufficient photoelectrons have also penetrated into the discharge tube in a light environment. These photoelectrons have the effect of exciting gas sealed in the discharge tube into an easily dischargeable state. Accordingly, the discharge tube placed in a light environment is in a stabilized and easily dischargeable state, thus causing a decrease in the initial discharge start voltage FVs. On the other hand, in a dark place, these photoelectrons do not exist, so that the discharge tube is unstable and it is difficult for discharging to occur, thus causing an increase in the initial discharge start voltage FVs.
The effect of thermoelectrons refers to the effect that the discharge characteristic of a discharge tube is made faster by thermoelectrons. That is, with an increase in temperature, an electron in the outermost shell of an atom becomes more likely to be emitted from the orbit of the outermost shell. Accordingly, the number of thermoelectrons generated also increases in the discharge tube as temperature increases. Therefore, the discharge tube is in a stabilized and easily dischargeable state in a high-temperature environment, thus causing a decrease in the initial discharge start voltage FVs. On the other hand, in a low-temperature environment, the number of thermoelectrons generated is reduced, so that the discharge tube is unstable and it is difficult for discharging to occur, thus causing an increase in the initial discharge start voltage FVs.
For the above-described reasons, the environment of −40° C. and complete darkness (hereinafter referred to as “dark cold environment”) is a harsh environment where it is difficult for the discharge tube 100A to cause discharging to occur. On the other hand, if a desired discharge characteristic can be obtained in this dark cold environment, a good FVs characteristic may be obtained in any environment.
Referring to FIG. 6 in view of the above-described matter, it is understood that in the discharge tube 100A, the initial discharge start voltage FVs increases as the cumulative number of discharges increases. This is because the discharge-inducing effects of thermoelectrons and photoelectrons on the discharge tube 100A completely disappear in the complete dark cold environment since the discharge tube 100A includes no sub discharge trigger wires contacting the metallized surfaces 40.
Thus, the discharge tube 100A has a problem in that the occurrence of surface corona discharge is delayed so as to reduce the response speed of the initial discharge start voltage FVs. Further, the initial discharge start voltage FVs increases with an increase in the cumulative number of discharges. In particular, the initial discharge start voltage FVs exceeds 1000 V around when the cumulative number of discharges exceeds 800,000, thus causing a problem in that the discharge life of the discharge tube 100A is reduced.
Meanwhile, FIG. 7 is a graph showing test results for the second conventional discharge tube 100B at the time of conducting the same discharge life test as described above. Test conditions and environment in this test are the same as those for the above-described discharge tube 100A.
FIG. 7 shows that in the discharge tube 100B, the initial discharge start voltage FVs fluctuates greatly as the cumulative number of discharges increases. The discharge tube 100B includes the sub discharge trigger wires 20 electrically connected to the metallized surfaces 40. Accordingly, even in the completely dark cold environment, induction of surface corona discharge is likely to occur in the discharge tube 100B compared with the discharge tube 100A with no sub discharge trigger wires 20. However, since the main discharge trigger wires 30 in the center are disposed at an irregular interval (or too wide an interval), transfer to main discharge is likely to be delayed, thus causing the initial discharge start voltage FVs to fluctuate as in the test results.
Specifically, referring to FIG. 5, the sub discharge trigger wires 20 are disposed between a group of the main discharge trigger wires 30 indicated by arrow A1 and a group of the main discharge trigger wires 30 indicated by arrow A2. Accordingly, the interval between the main discharge trigger wire 30 indicated by arrow MT1 of one group and the main discharge trigger wire 30 indicated by arrow MT2 of the other group is 2×W. Thus, the main discharge trigger wires 30 are disposed at an irregular interval in some parts, causing the initial discharge start voltage FVs to fluctuate.
Thus, fluctuations in the initial discharge start voltage FVs cause the operation of a ballast circuit for lighting an HID lamp using the discharge tube 100B to be unstable. However, compared with the FVs characteristic of the discharge tube 100A, the initial discharge start voltage FVs is prevented from rising excessively in the discharge tube 100B.